To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing.
Remember that a two-inch breathing space between the insulation and the roof sheathing must be included to allow for ventilation. If I leave room, it's hard to fit more than R-13 without going to rigid foam board.
Not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.
You can put all the insulation on top of the roof deck, all the insulation on the underside of the roof deck, or some on top and some below. You can put it all below the roof deck but use two different kinds. Your insulated roof can be a cathedral ceiling or it can be above a conditioned attic.
In roofs, the air gap should be larger, typically around 50mm (2 inches), to allow for proper ventilation and prevent condensation. It's important to note that some insulation materials, such as spray foam, do not require an air gap.
Insulation that touches the rafters provides better coverage and reduces the risk of heat loss. By creating a tight seal between the loft insulation and the roof, energy efficiency is improved, resulting in lower energy consumption and reduced utility bills.
This should be confirmed in the membrane manufacturer's BBA certificate, though 25mm to 50mm is typical.
A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.
Yes, too much loft insulation can cause dampness in some situations. Loft insulation works by trapping warm air inside a building, preventing it from escaping through the roof. This helps to keep the interior of the building warmer in cold weather, reducing the need for heating and saving energy.
Most building codes (including the International Residential Code) allow just one exception to these rules: in Climate Zones 2B and 3B, you can install fiberglass insulation against the underside of your roof sheathing without a ventilation channel or rigid foam above the sheathing — but only if you install tile ...
Depending on the specifics of the roof, it may be possible to fully fill the rafter depth or, in most cases, leave a 25mm or 50mm air gap between the top face of the insulation and the sarking membrane.
The general advice we give is that you should always insulate between and above the rafters (warm roof) or between and under the rafters (cold roof).
Best roof insulation for roof rafters
Place insulation material between or over the sloped structural beams (rafters) to insulate the roof. Creating a thermal barrier, roof rafter insulation keeps interiors warm in winter and cool in summer, benefiting living spaces.
Overall, the second layer of insulation plays a crucial role in improving insulation performance, reducing heat loss, and maintaining a comfortable and energy-efficient living space.
Unless otherwise specified by the material manufacturer, or this code, siding shall have a clearance of at least 6 inches (152 mm) from grade and at least 1/2 inch (13 mm) from other adjacent surfaces (decks, roofs, slabs).
airspace between the sheathing and the insulation. The IRC calls for a minimum of 1 in. of airspace; the AccuVent baffles shown here comply.
To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing.
The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space. While you might think that too much insulation can't be bad, there are some significant consequences to consider.
Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.
Yes, you should leave an air gap between the insulation and the roof to allow proper ventilation and prevent moisture buildup.
Roof and ceiling insulation. Installing roof and ceiling insulation can save up to 45% (or more) on heating and cooling costs.
An air gap will need to be 50mm wide, and the best way to ensure this is to attach timber battens of a 50mm width within your roof, which you can then place the insulation boards against. Use tantalised roofing battens of 50mm. These should be attached to the roof joists using a screwdriver or a nail gun.
You definitely don't want any insulation to touch the roof membrane as it'll pull water inside. Netting shouldn't be necessary - if the insulation doesn't touch when it is installed it'll only settle in time and won't touch later.
(Source: NRCA.) In the event you are erecting a “cold building” (such as a refrigeration building) that stays at 32 F (0 C) degrees or less inside, you will need a vapour barrier on the outside of the insultation to prevent warm exterior air from entering and potentially damaging the roof's insulation. (Source: NRCA.)